So I have a 2007 Volkswagen Rabbit with 85,000 miles. The car has the 2.5 5 cylinder engine and is a manual. I have’t had the car very long (put on less than 1k) and it’s having a problem with the cooling system. The car will sometimes refuse to reach the middle of the temp scale, which is were it should be. It will take a long time to heat up and will stop just before the middle and sometimes will get close to the middle marking in between the cold and middle. Now it doesn’t always do this. It started late this summer and then went a away for about a week and now it’s back and I’m trying to get to the bottom of it. The coolant fans are not constantly running either.
When I first noticed the problem I thought it was a simple thermostat (which is not so simple on VW) and I replaced that. It continued to do it however and so I did the top coolant sensor. This did not fix the problem either. Now there are two coolant sensors, one on the top of the engine and one near the bottom. I have not replaced the bottom one.
I was just wondering if anyone has had this problem or knows anything about it.

Not that I know of. I lost a lot of coolant with the thermostat and when adding it back I just would add a little and then start it until it went down enough to add some more, I guess I didn’t see any specific way to do it.

Did you test the two thermostats - the old and the new ones – in a pot of water on the stove to see at what temperature they open, and if they open to the dimension spec’d? If not , that’s what I’d do first if I had this problem. Also while the thermostat is out, make sure there’s nothing blocking its free movement that has somehow lodged in the cooling system near the thermostat housing.

Coolant sensors are usually just variable resistors, so there’s no need to replace them. Just use a dvm to make sure the sensor resistance in within spec. Check the condition of the connectors at the same time.

You’ve read the diagnostic codes with a scan tool and there’s nothing, right? You may have a problem with the computer and a code might be posted.

re: air bleeding. It varies from car to car but for something like a Rabbit I expect if you just open the radiator cap when the engine is cold and let the coolant heat up while idling in the driveway, topping the radiator off if it start to go down, that should work. When you do this, make sure all the coolant passages are open, like the heater is on max heat, etc. When you do this, that’s a good opportunity to double check the thermostat. What should happen is little to no coolant should pass into the radiator from the top radiator pipe at first. The all of a sudden a whole lot of coolant should come down that pipe, like if you turn a garden hose on full. That’s when the radiator thermostat opens.

@insightful
I don’t notice any change in heat coming from the vents. It seems to be normal, if I turn it to hot it feels hot and cold feels cold. In the middle it seems to be a little warm but this may be normal? Like I said, I haven’t had the car for too long.

@GeorgeSanJose
I did not test the two thermostats unfortunately. I do have the old thermostat still and could test that. I didn’t notice anything blocking the the thermostat either. The thermostat is not the easiest to get to either. You have to take off the whole intake manifold. I have gotten check engine lights irregularly with a code of “Cooling System Performance”.

I have also thought that it might be the wires going to the top coolant sensor. To get anything done with the engine you have to take this cover off which is right above the sensor and might have over time rubbed it.

I don't notice any change in heat coming from the vents. It seems to be normal, if I turn it to hot it feels hot and cold feels cold. In the middle it seems to be a little warm but this may be normal?

Sounds normal. I’m taking this to mean that even during the initial “slow” heat-up, the air from the vents gets hot much faster. If so, your wiring/electrical/connector theory sounds probable. Do you know for sure which sensor drives the dash gauge?

@insightful
I’ll have to check that tomorrow and get back to you on that. And I’m not sure which which sensor control the dash gauge. It sounded to me like the coolant sensor more drove the coolant fans (which again, are not running constantly and seem to be running normally).

Most cars I have owned keep the coolant temp gauge at about 40% of gauge scale. Your cooling fans are supposed to come on only when needed. I am not sure you have a problem. You could have someone with an infrared temp gun read your radiator when the car is fully warm. What degree thermostat did you put in?

I don’t think there’s any way the computer can reduce the engine coolant temperature by itself, other than turning on the engine compartment cooling fans. That function is left up to the thermostat. Since the fans aren’t coming on when this happens, unlikely to be a computer or sensor problem.

It seems the most likely culprits are

the new thermostat is not good just like the one you took out or there’s something preventing it from mechanically operating

the dash gauge is incorrect

If dash gauge display is determined by one of the two sensors you mention, then I’d have to take back what I said above. It could be the sensor that is causing this. Resistance-type temp sensors are an uncommon failure item, and are usually simple to check.

My Corolla has its own sensor exclusively for the dash gauge, and the computer isn’t involved with the dash coolant gauge function.

The idea above to independently check the coolant temp is a good one.

If the coolant temperature is in fact too low, that would usually show up as decreased overall mpg. Have you noticed that?

@insightful
So I checked to see if heat came out of the vents before it said it was hot. Now I don’t know how this works on other cars, but what I found was there was heat coming from the vents before the temp gauge even seemed to register and it seemed relatively warm/hot. It was probably another 2-3 minutes before it even started to register heat and probably 4-6 before it was a notch below the middle marking.

Yup, that’s exactly how to test the T-stat. Be sure the thermocouple you use is suspended in the water and not reading against the side of whatever vessel you use. Also suspend the T-stat. That’ll give you more accurate readings.

One other thing you might do is “T” in an autonomous temp gage to see what it reads relative to the dash gage. That might be revealing.

Bleeding air out of the system actually means getting an opening such as the fill cap to be the highest point in the system, filling, running, and filling again. Be sure the heater is on so that the valve to the heater core is open (if you have one, not all cars do) and the air bleeds from the heater core. Some cars have a burp valve in a high spot in the coolant circuit that might not otherwise purge through the radiator cap.